5-FU antitumor agents, such as 5-fluorouracil (hereafter, referred to as “5-FU”), a combination drug comprising tegafur and uracil, or a combination drug comprising tegafur, gimeracil, and oteracil potassium (a compound drug containing tegafur, gimeracil, and oteracil potassium at a ratio of 1:0.4:1 by mole, hereafter referred to as “S-1”), are generally used in treatment of various types of cancer, including digestive system cancer and non-small-cell lung cancer. Thymidylate synthase (hereafter, referred to as “TS”) involved in DNA synthesis is known as a target molecule of 5-FU. The correlation between the TS expression level and sensitivity to 5-FU antitumor agents has heretofore been reported as a result of many clinical trials. Specifically, the effects of 5-FU antitumor agents are remarkable for cancer patients exhibiting relatively low TS expression levels, although many cancer patients exhibing relatively high TS expression levels have resistance to 5-FU antitumor agents (Patrick G. Johnston et al., Cancer Res., 1995; 55: 1407-12; Kun-Huei Yeh et al., Cancer, 1998; 82: 1626-31). Accordingly, development of a novel cancer treatment technique for potentiating the antitumor effects of 5-FU antitumor agents is awaited for cancer patients exhibiting high TS expression levels and having resistance to 5-FU antitumor agents.
It was also reported that TS expression could be suppressed with the utilization of RNA interference (hereafter, referred to as “RNAi”), which had been developed as a tool for suppressing the expression of a given gene (JP Patent Publication (Kokai) No. 2005-253342 A). Accordingly, attempts to potentiate antitumor effects of 5-FU antitumor agents have been made through suppression of TS expression via RNAi, although the effects of potentiating antitumor effects are not yet satisfactory (Kadota et al., the 67th Annual Meeting of the Japanese Cancer Association, 2008, p. 235, P-4274).